Solid electrolyte sensor for monitoring the concentration of an element in a fluid particularly molten metal

ABSTRACT

The present invention relates to an electrochemical sensor for determining the concentration of a group IA metal in a fluid such as molten metal. The sensor comprises a substantially pure quantity of the group  1 A metal as a reference electrode ( 13 ) contained in a sensor housing ( 3 ), and a solid electrolyte constituting at least part of the sensor housing ( 3 ). The electrolyte is in electrical contact with the reference electrode ( 13 ) and the sensor is capable of operating at temperatures in excess of 973K. In a preferred arrangement, the sensor comprises a two part elongate conductor ( 15 ), a first part ( 17 ) of which extends from the reference electrode ( 13 ) into a refractory seal ( 11   a ), and a second part ( 19 ) of which extends from within the refractory seal ( 11   a ) externally of the sensor, the two parts ( 17, 19 ) being welded together.

[0001] The present invention relates to an electrochemical sensor fordetermining the concentration of an element (eg. a Group 1A metal suchas sodium or potassium) in a fluid, for example a molten metal. Themolten metal may, for example, be aluminium or an aluminium alloy, butthe invention is generally applicable to other metals and alloys, and toother fluids.

[0002] The invention in a first aspect is, however, preferably concernedwith the detection of sodium in molten aluminium or aluminium alloys,and although it will be appreciated that the invention is not limitedthereto, for convenience it will be described with specific reference tothese metals.

[0003] Sodium is often added to aluminium or aluminium alloys as astructural modifier in order to improve the physical properties of themetal. It is generally necessary to determine the concentration of thesodium in the metal melt so that the desired concentration may bearrived at.

[0004] Various designs of electrochemical sensor have been proposed inthe past. For example, UK Patent No. 1470558 discloses an apparatus fordetecting an element in a substance, in which a reference material is asolid electrolyte comprising a β-alumina compound of the element, or asolid compound of the element, such as a tungstate, molybdate orvanadate, separated from the substance by the β-alumina compound.

[0005] UK Patent No. 1602564 discloses a modification of the apparatusdisclosed in the above mentioned patent, in which a β-alumina compoundof the element to be detected is fused into the end of a tube ofrefractory material to provide a sealed tubular probe.

[0006] European Patent No. EP 0 679 252 B1 discloses a sensor for themeasurement of trace elements in molten metals or alloys which has asolid electrolyte formed from zirconia toughened strontium β-alumina.The sensor may, for example, act as a sensor for sulphur, in which caseit may incorporate a reference material comprising a mixture ofmolybdenum metal and molybdenum sulphide powders, which provides a fixedsulphur partial pressure against which the activity of the sulphur inthe molten metal is measured.

[0007] It is an object of the present invention to provide a sensorwhich obviates or mitigates one or more disadvantages of the knownsensors and which preferably offers one or more of the followingspecific advantages: durability, accuracy, repeatability and fastresponse time.

[0008] According to a first aspect, the present invention provides anelectrochemical sensor for determining the concentration of a group IAmetal in a fluid, comprising a substantially pure quantity of the group1A metal contained in the sensor as a reference electrode, and a solidelectrolyte providing at least part of the containment of the referenceelectrode, said sensor being capable of operating at temperatures inexcess of 973K.

[0009] The use of pure sodium as a reference electrode has been proposedfor low temperature applications, but it has not previously beenconsidered technically feasible to devise a sensor for use attemperatures above 973K (Zhang et. al. Metallurgical and MaterialsTransactions, 27B, 795, 1996).

[0010] The sensors according to the various aspects of the inventionoperate as Nernstian potentiometric cells, in which the solidelectrolyte separates the reference electrode, which has a knownchemical activity of the element (e.g. sodium) being measured(a_(El(Ref)), from the fluid (e.g. a molten metal or alloy) in which thesensor is immersed in use, which has an unknown chemical activity of theelement being measured (a_(El(Working))). The reversible electricalpotential of such a cell is governed by the Nernst equation, whichprovides the theoretical relationship between the potential (E) and therelative activities of the reference and working electrodes, as follows:

E=(RT/zF)*ln(a _(El(Ref)) /a _(El(Working)))

[0011] where:

[0012] E=the electrical potential (V)

[0013] R=the Molar Gas Constant (8.3144 Jmol⁻¹K−¹)

[0014] T=the absolute temperature (K)

[0015] z=the number of electrons transferred in the chemical systembeing measured

[0016] F=the Faraday Constant (96,485 Cmol⁻¹)

[0017] a_(El(Ref))=the chemical activity of the reference electrode (thesubstantially pure element)

[0018] a_(El(Working))=the chemical activity of the element in theworking electrode (the fluid)

[0019] A plot of the electrical potential (the sensor voltage) versusthe natural log of the ratio of the element activities of the referenceand working electrodes would yield a straight line with a Nernst slopeof (RT/zF). Since the electrical potential (the voltage) is measured bythe sensor, and the temperature of the fluid in which the sensor isimmersed (and hence the temperature of the sensor, including the solidelectrolyte and reference electrode) may be measured, the only unknownis the chemical activity of the element in the fluid, and this may becalculated from the Nernst equation. The concentration of the element inthe fluid may then be determined from the chemical activity of theelement in the fluid.

[0020] For embodiments of the invention in which the sensor determinesthe sodium (or other group 1A metal) concentration in the fluid, z=1,the activity of a substantially pure sodium (or other group 1A)reference electrode=1, and the Nernst equation for the system is asfollows:

E=(RT/F)*ln(1/a _(Na(working)))

[0021] It was mentioned above that a preferred element to be detected bythe sensor according to the first aspect of the invention is sodium.Consequently, the reference electrode preferably comprises substantiallypure sodium.

[0022] It was also mentioned above that the fluid in which the sensor isimmersed in order to determine the concentration of an element in thefluid, is preferably a molten metal (which term includes alloys).Particularly preferred molten metals include aluminium and aluminiumalloys (eg. Al.Si alloys).

[0023] The reference electrode is contained in the sensor (as part ofthe sensor), and at least part of the containment of the referenceelectrode is provided by the solid electrolyte. The solid electrolytepreferably defines a containment wall forming at least a part of ahousing, vessel or other container. The container formed (at least inpart) from solid electrolyte may, for example, be generally tubular inshape, with a closed end, for example, the container may be generallycup-shaped. The solid electrolyte material generally does not fullyenclose the space within the container, and the or each open portion ofthe container is preferably sealed by other means. Since the sensor ispreferably used in molten metal, it will normally be required towithstand elevated temperatures, and therefore the solid electrolyte andany sealing means for sealing the container are preferably formed fromrefractory materials.

[0024] A preferred material for the solid electrolyte is analumina-based material, and preferably a β-alumina material, such as aβ″-alumina material. Advantageously, the β-alumina material may betoughened (eg. against thermal shock) by the incorporation of otherelements, and zirconia toughened β″-alumina is especially preferred. Themost preferred material for the solid electrolyte is zirconia toughenedsodium β″-alumina.

[0025] As already mentioned, preferred materials for sealing one or moreopen portions of the solid electrolyte containment wall of the containercomprise refractory materials. It is particularly preferred for thesealing material to comprise two or more oxides of the followingelements: aluminium, calcium, magnesium, barium, boron and silicon.

[0026] A second aspect of the invention provides a process for theproduction of an electrochemical sensor for determining theconcentration of an element (eg. a group 1A metal) in a fluid,comprising providing a sealed container, at least part of a containmentwall of which comprises a solid electrolyte, and electrolyticallyintroducing a substantially pure quantity of the element into the sealedcontainer by passage of ions of the element through the solidelectrolyte containment wall.

[0027] The electrolytic introduction of the element into the sealedcontainer is preferably carried out by placing the container in a sourceof the element, in the case of a metal preferably a molten salt of themetal such as a nitrate, nitrite or hydroxide, or mixtures thereof,nitrite being preferred for safety reasons; a voltage is then appliedacross the solid electrolyte containment wall by means of a firstelectrical conductor which extends into the sealed container (theconductor is sealed into the container, for example by means of therefractory material referred to above) and which is in electricalcontact with the internal surface of the solid electrolyte containmentwall, and by means of another (second) electrical conductor which isimmersed in the source of the element. This potential difference causesions of the element to migrate through the solid electrolyte and intothe sealed container. This is particularly useful where sodium is theelement, for example, since it is a safe and effective way ofintroducing a precise quantity of sodium into the container (the firstelectrical conductor serving as a negative electrode). As analternative, the first conductor need not extend into the container, butcan be, for example, bonded or otherwise secured to that part of thecontainer not constituted by the solid electrolyte.

[0028] In the case where the electrical conductor extends into thesealed container, it is preferably an elongate electrical conductor, andis preferably formed from one or more metals (which term includesalloys) such as platinum, niobium or nichrome. In order to enhance theelectrical contact between the electrical conductor and the solidelectrolyte containment wall inside the container (and hence tofacilitate the electrolytic introduction of the group 1A metal into thecontainer), some preferred embodiments of the invention include aconductive (electronic or ionic) substance located inside the containerand in contact with the solid electrolyte and the elongate conductor. Apreferred electrically conductive substance is carbon, especially carbonfibre, for example one or more carbon fibre discs. Other suitableconductive materials include silicon carbide, β-alumina powder, TiO₂ andgraphite. Preferably, the atmosphere within the container isnon-oxidising (especially when carbon is used) to prevent oxidation ofthe metal reference electrode material and carbon when present. As aresult, the container contains little or no oxygen. For example, thecontainer may contain a vacuum, but preferably it contains an inert(non-oxidising) gas, for example argon or nitrogen.

[0029] It will be understood that operation of the sensors of thepresent invention requires a counter electrode. The counter electrodefunctions as an electrical conductor when immersed in the fluid in orderto enable the measurement of the electrical potential between the fluidand the reference electrode due to the difference in the chemicalactivities of the reference electrode and the metal being measured inthe fluid. The counter electrode preferably is electrically insulatedfrom the solid electrolyte to prevent short-circuiting (i.e. there is nodirect electrical contact between the counter electrode and the solidelectrolyte, the necessary electrical contact being via the fluid.

[0030] The counter electrode may form an integral part of the sensor.This arrangement enables the sensor to have a compact design, and isparticularly useful, for example where the sensor is in the form of aprobe to be dipped into the fluid. Preferably, the counter electrode isin the form of a ring or sheath surrounding part of the solidelectrolyte container. The counter electrode may, for example, be bondedto the solid electrolyte by means of an electrically insulatingadhesive, e.g. a ceramic cement. Alternatively, the counter electrodemay be separated from the solid electrolyte by an electricallyinsulating (eg. ceramic) sleeve to which it is secured, the sleeve beingsecured to the solid electrolyte. Advantageously, the counter electrodemay be formed, at least in part, from carbon (e.g. graphite).

[0031] In some preferred embodiments of the invention, the counterelectrode may comprise an elongate housing for the solid electrolyte,with the solid electrolyte being located at a first end of the elongatehousing, and the opposite end of the elongate housing arranged to beheld outside the fluid (e.g. molten metal) while the first end is dippedinto the fluid in order to determine the concentration of an element inthe fluid. Alternatively, for example, the counter electrode (as well asthe solid electrolyte) may be located only at the first end region of anelongate housing formed from one or more high temperature resistantmaterials, and the opposite end of the housing may be held outside thefluid. Suitable high temperature resistant materials include ceramicmaterials (e.g. ceramic fibres), silicon carbide and certain metals(e.g. steel encased within ceramic fibres or otherwise coated to preventdissolution of the steel), since the housing needs only to besufficiently temperature resistant to withstand the temperature of thefluid being tested (and, for example, a steel housing may generally besuitably temperature resistant where the fluid is aluminium or analuminium alloy). A particularly preferred arrangement is one in whichthe solid electrolyte and the counter electrode are retained at a firstend of an elongate metal (e.g. mild steel, nickel-plated to preventoxidation) member (preferably a tube), and the elongate metal member issurrounded, along at least part of its length, by a ceramic sheath(preferably formed from ceramic fibres).

[0032] Where an elongate metal member is used, this may convenientlyprovide an electrical connection between the counter electrode and avoltmeter used to measure the electrical potential across the solidelectrolyte. An elongate electrical conductor may extend through theelongate housing from the interior of the solid electrolyte to thevoltmeter, for example.

[0033] It will be understood that in alternative arrangements, thecounter electrode can be a separate component to the sensor itself, i.e.the counter electrode can be remote from the sensor, the two componentsbeing electrically connected in use. In some embodiments, particularlywhere the fluid is a molten metal, the counter electrode can beconstituted by a conductive inner lining of the vessel in which thefluid is contained.

[0034] A third aspect of the invention provides a durableelectrochemical sensor for determining the concentration of an elementin a fluid, comprising a sealed container containing a quantity of theelement or compound of the element as a reference electrode, at leastpart of a containment wall of the container being formed from a solidelectrolyte, and an elongate electrical conductor which comprises afirst portion formed from a first electrically conductive material and asecond portion formed from a second, different, electrically conductivematerial, wherein the first portion is in electrical contact with thereference electrode and extends from the reference electrode to within aseal of the container, and the second portion extends from within theseal to the outside of the container, the first and second portionsbeing in electrical contact with each other.

[0035] The third aspect of the invention has the advantage that thefirst electrically conductive material of the elongate conductor may bea material which is capable of withstanding contact with the material ofthe reference electrode, whereas the second material of the elongateconductor may be a material which is inert in air. For example, wherethe reference electrode comprises sodium under a non-oxidisingatmosphere, the first electrically conductive material may be niobium ora niobium alloy, since niobium is resistant to sodium, whereas thesecond material may for example be a metal which is inert in air, suchas platinum or a platinum alloy (e.g. platinum/rhodium). Niobium isoxidised in air, hence it would be unsuitable for use outside thecontainer, and platinum is attacked by sodium, hence it would be lesssuitable in terms of durability for use inside the container.

[0036] The seal of the solid electrolyte container is preferably arefractory material, more preferably a calcium aluminate-based material.

[0037] The sensors according to the first or third aspects of theinvention may be used, for example, in a process of adding an element(for example sodium) to a fluid (for example a molten metal, especiallyaluminium or an aluminium alloy), in order to determine when therequired amount of the element has been added to the fluid.

[0038] A fourth aspect of the invention consequently comprises a processfor the controlled addition of a predetermined amount of an element to amolten metal, comprising the steps of:—

[0039] (i) adding a quantity of the element corresponding to thepredetermined amount to the molten metal,

[0040] (ii) monitoring the actual quantity of the element achieved inthe molten metal using a sensor in accordance with the presentinvention,

[0041] (iii) adding further quantities of the element until the levelmeasured in step (ii) corresponds to the predetermined amount.

[0042] It will be understood that the level of element (eg. sodium)achieved in the molten metal (eg. aluminium) is likely to decrease overa period of time and be less than the quantity initially added becauseof losses by, for example, evaporation. For example, in a castingoperation, the level of sodium at the time of casting will be less thanin the ladle. The sensors of the present invention have a fast responsetime and allow the real-time monitoring of the quantity of the elementbeing detected. Thus, the output from the sensor can be used as feedbackto allow the amount of the element added to the molten metal to becontinuously varied so as to maintain the level in the molten metal atthe predetermined value.

[0043] A fifth aspect of the invention comprises an apparatus for addingan element to a fluid, the apparatus including a sensor according to thefirst or third aspects of the invention.

[0044] Embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

[0045]FIG. 1 is a cross-sectional illustration of a detail of a sensoraccording to the invention,

[0046]FIG. 2a shows a view of a sensor according to the presentinvention,

[0047]FIG. 2b is a detail view of part of the sensor shown in FIG. 2a,

[0048]FIG. 3 is a cross-sectional view of part of another sensor inaccordance with the present invention,

[0049]FIG. 4 is a schematic representing the electrolytic filling of asensor in accordance with the present invention,

[0050]FIG. 5 is a comparative plot of sodium concentration against timeas determined by a sensor in accordance with the present invention andas determined by spectrometric analysis for a sodium containing Al.Simelt,

[0051]FIG. 6 is a plot of sensor voltage against time for two similarsensors in molten Al.Si alloy containing sodium,

[0052]FIG. 7 is a plot of sensor voltage against time for four similarsensors in molten Al.Si alloy,

[0053]FIG. 8 is a plot of sodium concentration against time as measuredby a sensor in accordance with the present invention with variablesodium additions to an Al.Si melt, and

[0054]FIG. 9 is a plot of sodium concentration against time asdetermined by a sensor in accordance with the present invention for asodium containing Al.Si melt.

[0055]FIG. 1 shows a detail of a sensor 1 according to the invention,for determining the concentration of sodium in molten aluminium. Thesensor 1 comprises a container 3 formed from zirconia toughened sodiumβ″-alumina solid electrolyte. The container 3 is in the form of agenerally cup-shaped vessel, i.e. it comprises a tube have a closed end5 and an open end 7 which is sealed by means of a refractory tube 9formed from α-alumina and an inner seal 11 a formed from calciumaluminate refractory material. Sealing is effected between the outercircumference of the refractory tube 9 and container 3 by an outer(annular) seal 11 b also formed from calcium aluminate refractorymaterial. The container 3 consequently is hermetically sealed andcontains argon gas (rather than air) above the sodium (as indicated byreference numeral 12). The sealed container contains a substantiallypure quantity of sodium 13 which acts as a reference electrode; thesodium has been introduced into the container 3 electrolytically, asdescribed below. The container also contains a plurality of carbon fibrediscs (not shown) which facilitate the electrolytic introduction of thesodium.

[0056] Extending into the sealed container 3 from its exterior is anelongate electrical conductor 15 for providing an electrical connectionbetween the sodium reference electrode and a voltmeter (not shown). Theelectrical conductor 15 comprises a first portion 17 formed fromniobium, this first portion extending from the sodium referenceelectrode 13 to within the refractory seal 11 a, and a second portion 19formed from platinum, the second portion 19 extending from within therefractory seal 11 a to the exterior of the container 3. The first andsecond portions 17,19 of the electrical conductor 15 are joined together(by welding) within the refractory seal 11 a. As described earlier, theniobium is resistant to chemical attack from the sodium (but would beoxidised in air) and the platinum is inert in air but would be attackedby the sodium. In addition, the niobium has a comparable thermalexpansion coefficient to the calcium aluminate seal 11 a, producing athermally cycleable hermetic seal, and thus reducing the possibility ofsodium ingress into the seal 11 a. It should be noted that an oxideinterface exists between the niobium and the seal 11 a, and this is alsochemically resistant to sodium. The sealed container 3 and electricalconductor 15 will hereinafter be referred to as the “sensor head”.

[0057]FIG. 2b shows a sensor assembly according to the invention (novoltmeter or other ancillary electrical equipment, such as a computer,are shown). FIG. 2a shows an enlarged detail of the sensor shown in FIG.2b, in which the electrolytic container 3 of the sensor head shown inFIG. 1 is surrounded by a counter electrode 21. The counter electrode 21is formed from graphite and is in the form of a sheath surrounding partof the sensor head while leaving an end region of the sensor headexposed so that it may come into contact with the molten aluminium inuse. The graphite sheath is bonded to the exterior of the container 3 byelectrically insulating ceramic cement 23 and is stepped to form aregion having a relatively large outer diameter and a region having arelatively narrow outer diameter, an annular abutment surface 21 a beingdefined therebetween. The region of relatively narrower diameter isprovided with an external screw thread 24. The screw-threaded counterelectrode 21 is threadably attached to a first end of a correspondinglythreaded steel tube 25 such that the steel tube 25 abuts the annularabutment surface 21 a of the counter electrode 21 and an electrical leadwire 26 made of nickel (which is enclosed in insulation 28) which iswelded at a free end to the platinum portion 19 of the conductor 15extends through the interior of the steel tube 25. It will be understoodthat the conductor 15 could be made sufficiently long to extend throughthe steel tube 25, but nickel is less expensive than platinum. Theinsulation 28 protects the wire 26 from heat and possible oxidation atelevated temperature. It will be understood therefore that the steeltube 25 is in good electrical contact with the counter electrode 21. Thesteel tube 25 is itself surrounded by an outer ceramic fibre sheath 27,and the steel tube 25 and ceramic fibre sheath 27 together constitute anelongate refractory housing 29. The ceramic fibre sheath 27 rests on theannular abutment surface 21 a of the counter electrode 21, and a seal isformed therebetween by a bead of ceramic insulating cement 30. Thesheath 27 is a push fit over the metal tube 25 and is held in place bymeans of the ceramic cement bead 30. The entire housing 29 is shown inFIG. 2b, from which it can be seen that the ceramic fibre sheath. 27surrounds the steel tube 25 for only part of its length, a region 31 ofthe steel tube 25 remote from the sensor head being exposed because theceramic fibre sheath 27 is not required in this region 31 since thisregion 31 will not be immersed in the molten aluminium. An electricalcontact wire connected to the steel tube 25 (and therefore the graphitecounter electrode) and the lead wire 26 are indicated by referencenumeral 33. These wires are connected to a voltmeter (not shown) and itwill be understood that when immersed in molten aluminium, an electricalcircuit is completed.

[0058] Referring to FIG. 3, a modified sensor assembly is shown. Thesensor head 40 is as described with reference to FIG. 1. The sensor head40 is a close sliding fit within an alumina insulating ceramic sleeve42, an end of the sensor head being exposed. The sleeve 42 is secured tothe sensor head 40 by means of an annular bead 44 of insulating ceramiccement which also serves to prevent ingress of molten aluminium in use.

[0059] An annular carbon counter electrode 46 having an internal screwthread 48 is threadingly engaged onto an end of a thin wallednickel-plated mild steel tube 50 having a corresponding external screwthread 52. A ceramic fibre sheath 54 is a push fit over the metal tube50, the sheath 54 and carbon electrode 46 being of substantially thesame diameter. A thin layer of ceramic cement (not shown) is providedbetween the carbon electrode 54 and the ceramic fibre sheath 54 toprevent ingress of molten alumnium. The sensor head 40 and insulatingceramic sleeve 42 assembly is located within the steel tube 50 such thatthe cemented end of the sleeve 42 (and the exposed end of the sensorhead 40) projects beyond the carbon electrode 46. The sensorhead/insulating sleeve assembly is held in place by insulating cement 56applied through a pair of drillings 58 provided on a diameter throughthe carbon electrode 46.

[0060] The embodiment described with reference to FIG. 3 has twoimportant advantages over that described in relation to FIG. 2a:

[0061] 1. The sensor head 40 and counter electrode 46 are separated byan insulating sleeve 42 which is more effective in insulting electricalcontact between the sensor head 40 and the counter electrode 46. Unlikecement, the sleeve 42 is not prone to being worn or washed away.

[0062] 2. A relatively large diameter carbon electrode 46 is employed.In use, under the stringent operating conditions, the carbon electrode46 tends to crumble. The provision of a large electrode significantlyextends the sensor life.

[0063] The filling of the sensor with sodium is effected on the sensorhead 40 prior to assembly with the various holder arrangements.Referring to FIG. 4, the sensor head 40 is first weighed and the lead 26in electrical contact with the solid electrolyte is connected to thenegative terminal of a DC power supply 60. An accurate shunt resistor 62is connected in series between the DC power supply and the sensor head40 so that the charge current can be accurately measured during thefilling process. A steel wire electrode 64 is connected to the positiveterminal of the DC power supply by a second lead 66. The sensor head 40and steel electrode 64 are immersed in a heated bath 68 of molten sodiumnitrite (mp 271° C.) which is equipped with a thermocouple (not shown)to accurately monitor the bath temperature. A eutectic mixture of sodiumnitrate and sodium nitrite (32:68 mol %) can also be used, allowingfilling to take place at a lower temperature (226° C.) and a voltage andcurrent are applied across the sensor head 40 and the steel electrode 64until the charging current reaches a desired level. The sensor isconveniently filled in a constant current mode at a current of between50 and 100 mA. Typically about 0.1 to 0.2 g of sodium is filled.

[0064] During filling, the current, voltage and temperature are loggedand the quantity of sodium added is calculated from the integratedcharge current. After residual salt has been removed from the externalsurfaces of the sensor head 40, the sensor head 40 is reweighed asconfirmation of the calculated amount of sodium added.

[0065] The accuracy, response time and reproducibility of the sensorheads filled according to the above method were then assessed. In alltests the sensor head was preheated prior to immersion in the melt toavoid thermal shock and the possibility of fracture. It is known thatsubcritical damage can occur with β-alumina ceramics if they are exposedto thermal shocks of greater than 200° C. Although auxiliary pre-heating(eg. using a gas flame) can be adopted, it was found to be moreconvenient to use the radiant heat from the melt itself. Thus, thesensor head was held approximately 10 mm above the melt for about twominutes, approximately 3 to 5 mm above the melt for a further minute andthen immersed slowly into the melt.

[0066] Test 1

[0067] Referring to FIG. 5 a quantity of sodium was added (point A) to astirred Al.Si7% alloy melt at 735° C. The concentration of sodium in themelt was measured at intervals using a spark emission spectrometer and asensor head as described with reference to FIG. 1 (the sensor head wascemented to an α-alumina holder and an α-alumina protection tube wasprovided around the lead wires from the sensor head). As can be seenfrom FIG. 5 the concentration of sodium within the melt diminished overtime and the values derived from the sensor (arrows A) were in goodagreement with those measured by the spectrometer (arrows B).

[0068] Test 2

[0069] Referring to FIG. 6, two sensors of the same design as that usedin test 1 were immersed in an alloy of the same composition and at thesame temperature as described for test 1. Sodium was added to the melt(point A) and the sensor voltages measured for one hour. As can be seenfrom FIG. 6, both sensors responded very quickly to the increase insodium concentration (<1 min) and the two sensors were in good agreementas the concentration of sodium gradually decreased due to evaporationlosses.

[0070] Test 3

[0071] Referring to FIG. 7, an Al.Si7% melt was stirred at 700° C. andtwo batches of sodium were added (points A) with a four hour intervaltherebetween. Four sensor heads were immersed in the melt (heads mountedon 60% α-alumina tubes) and the sensor voltages measured. As can be seenfrom FIG. 7, all four sensors were in close agreement and all foursensors responded rapidly to each of the sodium additions.

[0072] Test 4

[0073] The sensors of the present invention are useful at even highertemperatures than described above. Referring to FIG. 8, sodium additions(variable) were made to an Al.Si10% alloy at 800° C. The sensordetermination of sodium level (arrow A) was plotted against the sodiumlevel as determined by spectrometer (arrow B) in FIG. 8 with goodagreement being found.

[0074] In each of tests 1 to 4, the counter electrode was a remotecarbon electrode.

[0075] Test 5

[0076] Referring to FIG. 9, a sensor as described with reference to FIG.3 was used to measure the sodium concentration of an AlSi10% alloy at775° C. As with the previous tests, the sensor (plot A) was in goodagreement with chemical (spectrometer) analysis (plot B) and a rapidresponse was observed on addition of sodium (arrow A).

1. An electrochemical sensor for determining the concentration of agroup IA metal in a fluid, said sensor comprising a substantially purequantity of the group 1A metal as a reference electrode contained in asensor housing, and a solid electrolyte constituting at least part ofthe sensor housing, said electrolyte being in electrical contact withthe reference electrode, said sensor being capable of operating attemperatures in excess of 973K.
 2. An electrochemical sensor as claimedin claim 1, wherein said reference electrode comprises substantiallypure sodium or potassium.
 3. An electrochemical sensor as claimed inclaim 2, wherein said reference electrode comprises substantially puresodium.
 4. An electrochemical sensor as claimed in any preceding claimwherein the fluid in which said group 1A metal is to be detected is amolten metal or metal alloy, preferably aluminium or an aluminium alloy.5. An electrochemical sensor as claimed in any preceding claim, whereinthe sensor housing is generally tubular in shape, with a closed end. 6.An electrochemical sensor as claimed in any preceding claim, wherein thesensor housing has at least one open region which is provided withsealing material whereby to seal the reference electrode within thesensor housing.
 7. An electrochemical sensor as claimed in claim 6,wherein the sealing material comprises at least one refractory material,preferably comprising two or more oxides of the elements aluminium,calcium, magnesium, barium, boron and silicon.
 8. An electrochemicalsensor as claimed in any preceding claim, wherein the solid electrolyteis an alumina-based material, preferably a β-alumina material.
 9. Anelectrochemical sensor as claimed in claim 8, wherein the solidelectrolyte is zirconia-toughened sodium β″-alumina.
 10. A durableelectrochemical sensor for determining the concentration of an elementin a fluid, comprising a sealed housing containing a quantity of theelement or a compound of the element as a reference electrode, at leastpart of the housing being formed from a solid electrolyte, and anelongate electrical conductor which comprises a first portion formedfrom a first electrically conductive material and a second portionformed from a second, different, electrically conductive material,wherein the first portion is in electrical contact with the referenceelectrode and is chemically compatible therewith and extends to within aseal of the housing, and the second portion extends from within the sealto the outside of the housing, the first and second portions being inelectrical contact with each other.
 11. A durable electrochemical sensoras claimed in claim 10, wherein the reference electrode is asubstantially pure group IA metal and the first electrically conductivematerial is niobium or a niobium alloy.
 12. A durable electrochemicalsensor as claimed in claim 10 or 11, wherein the second material of theelongate conductor is a material which is inert in air.
 13. A durableelectrochemical sensor as claimed in claim 12, wherein the secondmaterial is platinum or a platinum alloy.
 14. A sensor as claimed in anypreceding claim, wherein the sensor is mounted within an insulatinghousing, such that at least a part of said solid electrolyte is exposed.15. A sensor as claimed in any preceding claim additionally comprising acounter electrode.
 16. A sensor as claimed in claim 15, wherein thecounter electrode is secured to the sensor housing and electricallyinsulated from the solid electrolyte.
 17. A sensor as claimed in claim16, wherein an insulating sleeve is mounted between the counterelectrode and the solid electrolyte.
 18. A sensor as claimed in any oneof claims 15 to 17, wherein the counter electrode is graphite or siliconcarbide.
 19. The use of a sensor in accordance with any one of claims 15to 18 for the determination of a group IA metal in a different moltenmetal comprising the steps of:— (i) dipping the sensor housing into themolten metal such that the solid electrolyte is in contact with themolten metal, (ii) immersing the counter electrode in the molten metal,and (iii) measuring the potential difference across the referenceelectrode and the counter electrode.
 20. The use as claimed in claim 19,wherein the counter electrode is integrally formed with the sensorhousing and steps (i) and (ii) are effected concurrently.
 21. The use asclaimed in claim 19 or 20, wherein the sensor is pre-heated prior tostep (i).
 22. A process for the controlled addition of a predeterminedamount of an element to a molten metal, comprising the steps of:— (i)adding a quantity of the element corresponding to the predeterminedamount to the molten metal, (ii) monitoring the actual quantity of theelement achieved in the molten metal using a sensor in accordance withany one of claims 15 to 18, (iii) adding further quantities of theelement until the level measured in step (ii) corresponds to thepredetermined amount.
 23. A process according to claim 22, wherein steps(i) and (iii) are automated and step (ii) is continuous, the output fromthe sensor in step (ii) directly controlling the additions of step(iii).
 24. A process according to claim 22 or 23, wherein the element issodium and the molten metal is aluminium or an aluminium alloy.
 25. Aprocess for the production of an electrochemical sensor for determiningthe concentration of an element in a fluid, comprising providing asealed housing, at least part of which comprises a solid electrolyte,and electrolytically introducing a substantially pure quantity of theelement into the housing by passage of ions of the element through thesolid electrolyte by means of a voltage applied between a firstelectrical conductor in electrical contact with the solid electrolyteand a second electrical conductor in electrical contact with a source ofthe element external to the housing, the first conductor not being indirect electrical contact with the source of the element.
 26. A processaccording to claim 25, wherein the element is a metal and the metalsource is a molten salt selected from a nitrate, a nitrite, or anhydroxide or mixtures thereof.
 27. A process according to claim 25 or26, wherein the element is sodium
 28. A process according to claim 26 or27, wherein the first conductor extends into the housing and is inelectrical contact with the solid electrolyte from inside the housing.29. A process as claimed in claim 28, wherein an electrically orionically conductive substance is additionally provided inside thehousing.
 30. A process as claimed in claim 29, wherein said additionallyprovided substance is selected from carbon, especially carbon fibre,silicon carbide, β-alumina powder, TiO₂ and graphite.